Digital communications systems conventionally use receivers that include, in cascade, demodulator means, equalizer means, decoder means, etc.
The function of the equalizer means is to combat intersymbol interference caused in particular by the presence of multiple paths that are static or non-static over time.
Many kinds of equalizer means are already known in the art.
Historically speaking, the first systems used to combat intersymbol interference were essentially “synchronous” linear transversal filters.
Other equalizer structures that have been used include recursive non-linear decision feedback equalizers (DFEs) in which data that has been decided on is injected into a backward filter constituting the recursive portion of the equalizer.
The above kind of equalizer structure is generally used for transmission channels with adaptation algorithms for adjusting the parameters of the structures, in which case equalization is carried out in two stages. During a first stage, the structure is controlled by known training sequences that are inserted into the frames that are sent and cause the equalizer algorithm to converge. During the second stage, the structure becomes self-adaptive, i.e. it controls itself on the basis of its own decisions.
However, using a training stage has a serious drawback; in particular, it corresponds to a loss of efficiency in terms of bit rate.
For this reason considerable research has already been conducted with a view to proposing blind equalizer systems whose structure is able to converge towards an optimum solution in a “self-trained” manner, i.e. without using training sequences.
Blind equalizers that use different adaptation algorithms and structures according to the severity of the transmission channel have recently been proposed in patent application FR 2 738 967 and in a paper by J. Labat, O. Macchi and C. Laot “Adaptive Decision Feedback Equalization: can you skip the training period?”, IEEE Transactions on Communications, Vol. 46, No. 7, July 1998.
In particular, when reception is difficult, those equalizers use a convergence structure that includes, in cascade, a purely recursive filter and a transversal filter, whereas when reception is easy, they function in a tracking mode that uses conventional DFE structures, the decision to switch from one mode of operation to the other being a function of the performance achieved at the equalizer output.
It should be noted that the reversible nature of the change from one of the above two modes of operation to the other means that those equalizers can always function with a configuration that corresponds to optimum performance. They can therefore operate according to their own decisions with no risk of divergence, unlike conventional DFE. This essential property enables them to adapt to severe channel fluctuations, and therefore makes them particularly suitable for non-stationary channels, such as mobile radio channels, ionospheric channels, and submarine acoustic channels.